The brains of (L-R) a non-avian dinosaur close to birds, Archaeopteryx and a modern bird not to scale). Modified from Balanoff et al. 2013.

Despite attempts to knock it from its perch, Archaeopteryx is still widely recognised by palaeontologists as the first bird. Effectively it is the first dinosaur that is closer to modern birds than to other dinosaurs and thus provides a clear picture of what an intermediate between the two would be like. It is not an intermediate in itself - that is, Archaeopteryx is not some animal that is literally halfway between the two, or was in the process of becoming a bird – but instead represents the closest genus we know of to that transition.

That it does have features in common with both birds and dinosaurs was a key part of the work that helped establish birds as dinosaurs. Among others, it has feathers just like a bird (including flight feathers), but has the clawed hands, long bony tail and teeth of a dinosaur. Modern research has revealed that many of these features are now known in both other non-avian dinosaurs and also various fossil birds. This is no surprise, evolution does not typically operate in leaps and bounds but gradual changes and so we would expect to find a continuum between earlier dinosaurs and later birds and this is clearly present in both the fossil record and reconstructed family trees of these animals.

Nevertheless, Archaeopteryx does seem to have some features that place it pretty much exactly between the non-avian dinosaurs and the birds and one of these is the size of its brain. Biologists typically use a simple scale called the encephalisation quotient to give an idea of the relative brain size of an animal and it relates brain volume to body mass. Larger animals need larger brains to function, but additional grey matter beyond that is likely to indicate a relative increase in brain function, be it intelligence or a specialisation in sensory data (e.g. great vision) or something else.

Determining the size of brain in a fossil reptile or bird is actually relatively easy. Their brains are encased in a part of the skull termed the braincase, that tightly fits around the brain, and the internal space of this gives a superb indicator of the size, and indeed shape, of the organ within. The hard part is actually getting to it, since obviously what we want to measure in the internal space, which is surrounded by the braincase, and the rest of the skull. This can be done with a broken skull where the braincase is exposed, but most often it's done by finding a near-perfect skull and running through a CT scanner to get a 3D representation of the space the brain occupied.

Both birds and mammals have pretty large brains for their size and stand out well above the norm for other vertebrates. Modern birds have brains at least six times larger than you would predict by their size alone based on other animals so they clearly have some decent processing power. Archaeopteryx however has a brain size that fits almost exactly between the two, far larger than that of earlier dinosaurs, but much smaller than more recent birds.

However, a new study shows that, like a great many other characters of Archaeopteryx, this is far more of a continuum with other forms that an outstanding feature. Scanning of multiple skulls of non-avian dinosaurs that are close to the origin of birds reveals that many of these had brains that were proportionally as large as, or were even larger than, that of Archaeopteryx. This doesn't diminish the mental prowess of the first bird, but it does suggest that several other relatives had similar faculties and that is intriguing.

Although Archaeopteryx is still considered the earliest animal capable of powered flight, a number of close non-avian relatives have at least some features that suggest they may have been capable of gliding or other passive forms of flight – trading height for distance and moving between trees as do animals like 'flying' frogs and 'flying' squirrels. That these other dinosaurs had similar brains suggests that, while Archaeopteryx may now not be that special in terms of brain size, it already had most if not all the brain required for flight before the lineage took to the air fully.

Interestingly though, the first bird does still seem to have a unique feature. The researchers identified a modification to the braincase that indicates the presence of a 'wulst' – a part of the brain unique to birds that assists in processing sensory input. None of the non-avian dinosaurs showed indications of the wulst, so while in other respects Archaeopteryx seems to be consistently losing its 'unique' features as other species are found that share them, there is a new one that appears to still tie it beautifully to the birds.